Many times during operations in a wellbore, objects become stuck and must be retrieved from the wellbore. Sometimes the casing or tubing can experience a catastrophic failure and must be retrieved from the wellbore. In the past, various mechanical designs have been used which generally involve a series of mechanically actuated grippers to slips to grab the object to be retrieved or "fish" so that it can be brought to the surface. Many of these designs employed shear pins that have to be sheared to allow release from the fish, if required. These tools were not resettable because once the shear pin was broken the fishing tool had to be brought to the surface so that the shear pin could be redressed. Additionally, the use of shear pins limited the upward pull that could be exerted on the fishing tool. Operators of fishing tools that had shear pins had to be careful not to exert too great a pulling force or else the fishing tool would accidently release the fish. Another drawback of shear pins was that they would release at smaller values of forces than anticipated. This was primarily due to the cyclical stresses imposed on shear pins which, over time, would weaken them and make them release or fail at pulling forces lower than anticipated.
Various tools, in the past, have employed different mechanisms to set the slips. Some have done so mechanically, while others have done so hydraulically. Typical of such tools are U.S. Pat. Nos. 808,378 (mechanically set); 803,450 (hydraulically set); 1,457,139 (hydraulically set); 1,728,136 (hydraulically set); 1,619,254 (hydraulically set); 1,580,352 (hydraulically set); 1,621,947 (hydraulically set); 1,638,494 (hydraulically set); 1,712,898 (hydraulically set); 1,779,123; 1,794,652; 1,815,462; 1,917,135; 2,141,987; 2,290,409; 2,806,534; 2,732,901; 3,638,989; and 3,262,501. Some of these tools employ hydraulic force to move a piston to in turn move a mechanical member which in turn sets the slips for gripping. Thereafter, some mechanical action is required to release the slips, such as breaking a shear pin or by pulling up on the tool with sufficient force. Also of interest is European Application 0213798, which discloses a packer retrieval assembly. This device presents two different outside diameters so that it can be inserted through a packer and expanded to its larger diameter for retrieving the packer. This apparatus also uses shear pins to actuate from one position to another. U.S. Pat. No. 4,616,721 shows a packer retrieval tool having a milling feature for cutting loose the slips. This tool can disengage the packer only by failure of a ring component from hoop tension. At that point, the packer falls to its original position and the tool must be removed from the well to be reset.
Also of interest to the field of this invention is a packer retrieving tool product No. 646-17 made by Baker Oil Tools and referred to as BAKER 43 RETRIEVA-D LOK-SET.RTM. which is used to retrieve BAKER 43 RETRIEVA-D LOK-SET.RTM. packers.
A fishing tool that releases hydraulically and which can release from the fish and reattach to the fish without removal to the surface is illustrated in U.S. Pat. No. 5,242,201.